In recent years, artificial bone grafting for treatment of a bone defect of a patient has received attention, since this therapeutic method imposes less physical burden on the patient than autologous bone grafting for treatment of the bone defect, and does not involve problems associated with preparation of autologous bone grafts.
A bioceramic material such as hydroxyapatite has been known as a good artificial bone material, since it chemically binds to bone. Unfortunately, such a bioceramic material has low strength and poor impact resistance.
A metal material having very high strength, such as a titanium alloy or a cobalt-chromium alloy, has also been known as an artificial bone material. However, such a metal material probably causes, for example, metal allergy, and has a much higher elastic modulus than biological bone. Thus, when artificial bone formed of a metal material is transplanted to, for example, a bone defect, stress shielding may occur due to the difference in mechanical characteristics between the metal material and biological bone, resulting in absorption of bone around the bone defect, and weakening of the bone.
In recent years, a resin (e.g., engineering plastic) has received attention as a material which can solve the aforementioned problems and has mechanical characteristics similar to those of biological bone. For example, high-density polyethylene resin is suitable as an artificial bone material, because of its very low elasticity. Polyether ether ketone (PEEK) is also suitable as an artificial bone material, since it has mechanical characteristics similar to those of biological bone, and exhibits excellent biocompatibility.
As is well known in the art, the structure of artificial bone is an important factor in terms of its binding ability to biological bone. Thus, a method has been proposed which transforms the surface of a resin molded product or the entire molded product into a porous structure so that a biological tissue readily enters the molded product when it is embedded into a living organism.
For example, Patent Document 1 describes “A polymer compound porous composite structure characterized by comprising a first sponge-like polymer compound porous structure including, in its pores and surface portions, a second sponge-like polymer compound porous structure which is formed of a polymer compound different from that forming the first sponge-like polymer compound porous structure.” (claim 1).
Patent Document 2 describes “A composition characterized by comprising a porous macro (gross) structure formed of a biodegradable polymer and having open voids, and a porous microstructure having a large surface area per unit weight and located in the open voids.” (claim 1) and “A composition according to claim 1, wherein the porous microstructure is formed of a chemotactic ground substance.” (claim 2).
Patent Document 3 describes “A biodegradable and biocompatible porous scaffold characterized by a substantially continuous polymer phase having a highly interconnected bimodal distribution of open pore sizes comprising rounded large open pores of about 50 to about 500 microns in diameter and rounded small open pores less than 20 microns in size, wherein said small pores are aligned in an orderly linear fashion within the walls of the large pores.” (claim 1) and “The small pores are formed when the polymer solution undergoes phase separation under cooling.” (paragraph 0037).
Patent Document 4 describes “A porous structure comprising a network of polymer granules that are melted together at contact points, a microporous surface structure on the granules, and a plurality of interstitial spaces between the polymer granules” (claim 24) and “The particles are mixed with β-TCP at a ratio of 90% polymer 10% β-TCP. The resulting powder mix includes polymer particles coated with β-TCP. The presence of β-TCP causes the particles to bead and to prevent flow at or above the melting point of the polymer. The final material defines an interconnecting porous polymer with β-TCP coating. In a selected embodiment, the β-TCP or other coating powder is later removed from exposed surfaces within pores via acid leaching, a selective solvent process, or another powder removal process. In this case, the surface is calcium poor.” (paragraphs 0024 and 0025).